New study explores efficacy of vasectomies in reducing feral cat populations

Flickr photo by hehaden

For the estimated 80 million domestic cats that are kept as house pets, there are as many roaming free.

Those numbers shouldn't be surprising, considering the rate at which felines are able to reproduce.

 I've previously written about the overpopulation issue with cats and dogs, and getting these numbers in check has been the focus of many. Spay and neuter programs and protocols are helping to make progress.


First, understand that from a biological standpoint, we are in a battle with pets.

Reproductive success drives evolution, pure and simple. It's the strongest biological factor in any species. Biology has a way of taking over, jumping any hurdle that is put in its path and compensating. The pets themselves have no control over their biological drives, and therefore can't curb their behavior when it comes reproducing.

Feral cat colonies are a supreme example of biology's stronghold.

Comprised of a clowder of free-roaming cats that are the descendants of unaltered tame cats somewhere in their ancestral line, the social structure is by no means random: at its core, it has at least one sexually-active dominant male and fertile females who are often well-bonded and who will help care for their respective litters and each other.

Colonies are often formed around shelter — be it a wooded area, abandoned house, under a porch area that doesn’t get that much foot traffic or something else — and a food source of some sort.

Because of their unique resiliency, feral cat colonies have posed a special challenge.
The structure and reproductive patterns of these groups have piqued the interest of researchers and got them thinking: Could the way that a feline in a feral colony is sterilized impact the overall numbers of new litters that are born?

A new study focusing on one method of sterilizing cats in colonies — trapping them, giving vasectomies or hysterectomies (versus ovariohysterectomy) and releasing them back into the colony (abbreviated TVHR) — offers some insight.

The results of the study, which simulated a cat population of roughly 200, were published in the Aug. 15, 2013 edition of the Journal of the American Veterinary Medical Association.

But why a computer-simulated study?

First, TVHR is not a common way to address feral cat populations. Trap, neuter and release (TNR) on the other hand, is a more widely-accepted approach to controlling feral cat colony populations, and for a lot of reasons.

Because TVHR isn’t put into use as much and because the life span of feral cats is far shorter — an average of three years as opposed to the 15 that their indoor counterparts enjoy — it’s been difficult to extrapolate the long-term data that helps to give some solid numbers that researchers would be looking for. Each computer run simulated the feral cat population over 6,000 days, tracking individual cats on a daily basis, thus predicting effectiveness of TVHR.

New cats were added to the population as they were born and cats that died were removed, creating a “family tree” of sorts.

But before talking about the results of the study, it’s probably a good idea to flesh out the differences between the two methods and the advantages to both.

Same goal, different approaches

Neutering a male cat entails removal of their testicles — thus leaving them not only infertile, but sexually inactive.

Those two things are very advantageous: the cats don't reproduce, and because they no longer produce reproductive hormones, behaviors like fighting, spraying and howling are reduced, addressing the needs of the community-at-large. (Behaviors like those would be troublesome to anyone who lives in close proximity to a feral colony.)

A possible advantage to vasectomy as opposed to neuter procedure is that though the tube that carries semen is cut, the animal retains their testicles and their reproductive hormones. For that reason, upon being returned to the colony, the cat preserves his dominant position and can continue mating with females without producing kittens — and quite possibly protect their turf from other male competitors that are “intact”.

Conversely, a neutered male loses his dominant position in the colony, and the next most dominant male takes his place — and the cycle continues. (It's important to note that when a female cat that has not been sterilized mates with a male that has had a vasectomy, she enters a 45-day pseudo-pregnancy, dipping the chance of fertile mating even further.)

The findings and commentary

Researchers discovered that with an annual capture rate of 35 percent using TVHR, the population would be cut in half and the entire colony would disappear in 11 years. To achieve the same results with TNR, 82 percent of cats would need to be captured and neutered.

Robert J. McCarthy, D.V.M., lead author on the study and clinical associate professor of small animal surgery at the Cummings School of Veterinary Medicine at Tufts University hopes that TVHR can be put to the test in a more broad sense, rather than the controlled environments and small colonies that it has been shown promise in.

“This opens up new conversations,” said McCarthy.

“The computer model indicates that vasectomy and hysterectomy should be much more effective at reducing or eliminating feral cat populations than the traditional approach of neutering. The next step is to gather evidence on how it actually works in the field.”

The topic of feral cat colonies and how to manage them is one that brings up a lot of emotion in many communities, and Washtenaw County is no exception.

One thing that is certain is that it’s going to take time in order to see a favorable result, no matter the approach to managing feral colonies.

And, some experts believe that a combination of methods would be advantageous, including Sheilah Robertson of the American Veterinary Medical Association's Animal Welfare Division.

"…a multipronged approach will be required that includes TNVR; programs that use nonsurgical approaches, including immunocontraception and chemical sterilization of male cats; and trap-and-remove. Regardless of the method chosen, it may take 10-15 years of sustained effort to see a positive effect," she said.

Click here to read more on the study from TuftsNow.com.

Lorrie Shaw is a blogger and owner of Professional Pet Sitting. Shoot her an email, contact her at 734-904-7279 or follow her adventures on Twitter.